I don't think that is possible on the free version. However, I believe you should be able to create a new team with the smaller set of users you want to share the board with and make that board a part of the new team.
I hope that helps!
Hello Community! My name is Brittany Joiner and I am a Trello enthusiast and Atlassian Community Leader. In this video, I'll share my favorite Trello templates. Templates mentioned in ...
Connect with like-minded Atlassian users at free events near you!Find an event
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no Community Events near you at the moment.Host an event
You're one step closer to meeting fellow Atlassian users at your local event. Learn more about Community Events